This invention relates to the manufacture of olefin polymers in fluidized beds, particularly to the monitoring and anticipation of conditions likely to result in upsets or runaway reactions which might necessitate a shutdown, and to corrective action to prevent such upsets or runaway reactions.
In a highly competitive market, olefin polymerization reaction systems are pushed ever closer to their operating constraints in order to increase production in existing equipment. Operating close to the constraints, the possibility of a reactor upset becomes greater. For purposes of this invention, an upset is a reaction which exceeds normal operating constraints to a degree which requires a rapid reduction in production rate to prevent a significant economic loss by way of a shutdown, production of unusable product, loss of raw material, or the like. Our invention seeks to identify an incipient upset which would otherwise require a shutdown or other drastic or unusual operating action unless an effective amount of polymerization retarding agent is introduced imminently. Typically an upset could involve a significant increase in temperature in the reacting system, which may escalate the reaction further out of control and/or dangerously soften the product particles, causing them to stick together and form large unmanageable agglomerates. Unusual pressure values, superficial gas velocities, or static effects can also be harbingers of upsets, although not as common or always as dependable as temperatures. Upsets can also be the result of influences that are not measured or predicted, such as variations in reactant feed quality, irregular catalyst flow(s), and equipment malfunction. To maintain reactor operation and avoid expensive shutdowns, polymerization retarders can be fed into the reactor during or following an upset once it is detected, but manual intervention to introduce fast acting polymerization retardants may often not be timely enough or accurate enough to avoid the adverse consequences of major process upsets. Human monitoring cannot be expected to suffice where combinations of rapidly changing variables must be interpreted to anticipate runaway conditions.
Fast acting mechanisms to retard polymerization rates are well known and may include carbon monoxide and other catalyst poisons. See EP 0,376,559 for examples of catalyst poisons and polymerization rate retarders. U.S. Pat. No. 4,306,044 proposes the use of CO2 to quench or retard a polymerization system when the reaction temperature reaches an unacceptable level. U.S. Pat. Nos. 4,834,947, 4,786,695, 4,431,855, 5,336,738, 4,786,695 and European Patent EP-471 497 B1 describe the introduction of activity retarders for terminating the reaction under emergency conditions to maintain the temperature in the reactor below the sintering temperature of the polymer. These emergency conditions are typically equipment failures whereby the heat generated from the exothermic polymerization reaction cannot be removed from the process, resulting in sintering of the resin bed. An activity retardant is introduced to the reaction system to completely terminate the reaction mechanism. U.S. Pat. Nos. 5,066,736 (EP 359 444) and 5,432,242 describe processes in which a very small amount of activity retarder is continuously introduced into a reactor at a flow rate which is varied in time so as to maintain a substantially constant polymerization rate. This more or less constant feeding of activity retarder prevents the achievement of maximum catalyst productivity and therefore increases productivity costs in the reactor and also tends to degrade product quality. In addition, the polymerization rate alone may not correlate with temperature excursions above the sintering temperature of the polymerized bed and many other typical process anomalies. Further, disruptions which can result in a reactor shutdown often do not affect production rate measurements in a manner timely enough to take appropriate action.
A novel process has been developed which provides early detection and identification of events likely to lead to upsets and automatically introduces a prescribed amount of polymerization retardant delivered over a prescribed period of time to keep the process within the operating envelope. The following generalized steps comprise the invention: (a) continuously, periodically or intermittently monitoring at least one condition in the system, the condition being useful as a harbinger of an incipient upset (b) continuously, periodically or intermittently comparing the monitored condition to a critical level thereof for the manufacture of the resin product beyond which an upset is likely, the critical level being determined continuously, periodically or intermittently by a model based on knowledge of the polymerization reaction system (c) upon determining that the condition exceeds the critical level, determining an amount and duration therefore for feeding retarding agent to said reaction system for the catalyst to return the condition to below the critical level, and (d) delivering the retarding agent into the reactor.
The invention includes the further optional steps of (e) continuously, periodically or intermittently monitoring the effects of the corrective action as it proceeds, (f) modifying the corrective action (retarder feed) to maintain prescribed operating conditions as the system reacts to the presence of retarder, (g) terminating the retarder feed when it is no longer needed, as determined at least partly by current inputs and (h) adjusting one or more process variables to compensate for long term effects of the presence of the retarder in the reactor and/or the reduced polymerization rate already effected. In addition to these further designated steps e-h, one must, in preparation, establish a computer model of the process capable of determining the likelihood that various combinations of values of the monitored and model variables will lead to an upset condition within the time frame of interest. The computer model is an embodiment of a priori knowledge which reflects the dynamic relationship between process measurements and a potential process upset; it comprises one or more algorithms periodically executed using real time process measurement or data.
This invention entails a novel automated process including introducing a measured quantity of polymerization retardant to a reaction system at a feed rate based on at least one process measurement or inferred variable and a comparison of that measurement or inferred variable to an acceptable range for that process measurement in a computer model for the polymerization process in use and under at least one other monitored prevailing condition. The process measurement(s) may include monitored reaction temperatures, electrostatic charge, and resin flowability. Comparisons may be based on a priori process knowledge such as sintering temperature of the polymer, tolerable electrostatic charge intensity and duration, resin density limits, and other factors derived from experience with the process and the product being made. The computer model will reflect historical experience preferably for at least two process control variables, including a variable to be controlled in the process, and an acceptable range or critical level for the controlled variable is preferably varied continuously or intermittently by monitoring and entering at least two model input variables as the process continues. Current practices require manual observation of process upsets and a manual initiation of a polymerization retardant. By using a fast acting automated controlled effort, dynamically responsive to ongoing and current variations in the system, major process upsets can be attenuated or avoided, resulting in a more consistent, improved product quality and increased production throughput.
It will be seen that our invention is a method of inhibiting the maturation of incipient upsets in a polymerization reaction system making a particulate polyolefin resin product in the presence of a polymerization catalyst, comprising (a) continuously, intermittently, or periodically monitoring at least one condition in the system, the condition being useful as a harbinger of an incipient upset (b) continuously, intermittently or periodically comparing the monitored condition to a critical level thereof for the manufacture of the resin product beyond which an upset is likely, (c) upon determining that the condition exceeds the critical level, determining an amount and feed rate duration therefore of retarding agent for the catalyst to return the condition to below the maximum, and (d) delivering the dose of retarding agent into the reactor. As a follow-up step, the level of catalyst feed may be reduced by at least 2% below its level at the beginning of step (c). At least one monitored condition may be a rate of increase of temperature monitored at a given point or points in the reactor. Thus our invention includes a method of inhibiting an imminent upset in a fluidized bed olefin polymerization reactor comprising introducing polymerization retarder as a function of the rate of increase of a temperature monitored in said reactor. One or more conditions may be prescribed in a computer model. By the maturation of an upset, we mean attainment of conditions whereby an upset is imminent and highly likely to be unavoidable without intervention.